This invention relates to a rapid, nonaqueous process for producing optically clear, inorganic gels of silicon, titanium and zirconium.
The above-described inorganic gels can be dried to form strong, hard, optically clear compositions that are useful as abrasion-resistant and controlled refractive index coatings in optical applications such as, for example, lenses and windows. They may be used as dielectric coatings and anti-corrosive coatings. Additionally, the gels have a pore structure of extremely small size, high specific surface area and narrow pore size distribution which makes them useful for catalyst or enzyme support, gas separation and in chromatography as column packing.
Inorganic gels of silicon, titanium and zirconium are typically created by hydrolysis of organo-silicon,-titanium and -zirconium compounds, and subsequent condensation of the hydrolysis products. For silica gels, hydrolysis, (I), and condensation, (II) and (III), are represented by the following equations: EQU --SiOR+H.sub.2 O.rarw..fwdarw.--SiOH+ROH (I) EQU 2(--SiOH).rarw..fwdarw.--SiOSi--+H.sub.2 O (II) EQU --SiOH+--SiOR.fwdarw.--SiOSi--+ROH (III)
where R is an alkyl group.
Hydrolysis and condensation reactions are normally catalyzed by acids or bases.
Existing methods of producing small pore size, high specific surface area, inorganic gels suffer from a number of problems. As discussed by C. Plank et al. in J. Colloid Sci., 2, 399 (1947) small pore, high surface area gel is ideally produced near the isoelectric point which for silica is about pH of 2. However, acid-catalyzed condensation reaction is so slow that several weeks or longer may be required for gelation to occur at about 25.degree. C. at this pH. If condensation is conducted at elevated temperature, complete gelation can occur in less time; however, pore size will increase and surface area will decrease. In general, alkaline or fluoride ion catalyzed gelation is faster but produces larger pore size gels than acid catalyzed gelation and may lead to opaque gels. Another problem is that many conventional processes use alcohol as solvent for reactants. Presence of alcohol enhances the reverse of reactions (I) and (III) which hinders production of gel.
U.S. Pat. No. 4,950,779 (Wengrovius, et al.) discloses a process for making organosilicon oligomers by reacting polyalkoxysilanes or polyaminosilanes with formic acid without addition of water. Wengrovius' products are organosilicon oligomers of low molecular weight, in contrast to gels of silicon dioxide which are completely crosslinked networks of unlimited molecular weight. Wengrovius' silicone oligomers contain at least one non-hydrolyzable organic substituent on each silicon atom which limits the ability of the oligomers to form gel.
Coltrain et al. demonstrate in a paper presented at a 1989 conference and published in Ultrastruct. Process. Adv. Mater., Wiley, New York, N.Y., 1992, pp. 69-76, that the rate of acid-catalyzed hydrolysis of tetraalkoxysilanes and formation of gels depends on the pH of the medium. For a strong organic acid such as trifluoroacetic acid, the rate of reaction in an aqueous medium slows down as the concentration of the acid increases.
It is desirable to provide a rapid process requiring only as much water as necessary to propagate reaction, for the synthesis of open pore, inorganic gels having fine pore structure.